Patent GB-1492979-A (GB)
Title: Phase-controlled cycloconverters
IPC: H02M5/27
Filed: 1974-06-03
Abstract: 1492979 Frequency changers NATIONAL RESEARCH DEVELOPMENT CORP 30 May 1975 [3 June 1974] 24430/74 Heading H2F  Cycloconverter apparatus comprises input terminals for a three phase A.C. supply of a certain frequency, output terminals providing an A.C. supply of frequency different from that of the input frequency, a sine wave generator to generate a reference wave of the output frequency, for each phase of the output supply, first voltage comparator means to decline first instants of amplitude equality of each input supply phase compared with each other supply phase, means to sample the voltage amplitude of each reference wave at said instants, voltage storage means to store each sampled amplitude during the next following 180 degrees at input supply frequency, thereby to provide a modified reference wave of step-form amplitude, switching means made conductive by firing pulses and controlling current flow from each input phase, for the negative and positive half cycles of each phase of the output supply respectively, second voltage comparator means to determine second instants of amplitude equality of each input supply wave and each modified reference wave, and pulse generating means to generate the firing pulses at the second instants. The cycloconverter comprises groups of positive and negative thyristors 1-18 connected with the input supply ABCN, reactors 22, 23, 24 and loads 25, 26, 27 as shown in Fig. 1. The modified reference waves are obtained from the circuit shown in Fig. 8 wherein, in the case of the # phase circuit illustrated, an operational amplifier 107 receives rectified inputs of the A and B phases to switch on a transistor 112 at the intersections of these phase waveforms, Fig. 7 (not shown), a further amplifier 108 receiving the sinusoidal reference wave and using it to charge a capacitor 115 each time transistor 112 switches on. The capacitor voltage is stored and applied through high impedance circuit 109, 113 to output 114 for 180 degrees period, whereby a stepped reference wave is obtained, Fig. 9(d) (not shown). These waves are applied to units (104, 105, 106) Fig. 6 (not shown) which generate firing pulses for the thyristors 1-6 of output phase I. Fig. 4 shows how the firing pulses are produced at the abovementioned second instants, the modified stepped reference wave (1) and an input supply wave (2) being supplied to a comparator 70, which produces pulses having durations determined by the intersections of the two waves and which supplies parallel circuits 76-80, 91, 92 and 81-86, 93, 94 including NOR elements and free-running oscillators, whereby firing pulses are produced by transformers 95, 96 for the positive and negative groups of thyristors respectively (in each output phase), positive and negative firing pulse trains being produced at the trailing and leading edges respectively of the pulses from comparator 70. Operation of Fig. 4 is explained, Fig. 5 (not shown), for a sinusoidal reference wave and using natural sampling, although the control of the invention uses the modified reference wave with regular sampling. The frequency and amplitude of the reference waves may be varied to correspondingly vary the frequency and amplitude of the cycloconverter output.